By Daniel Hartill,Staff Writer

It’s the rarefied air of few authors: creating a book so popular that even grocery stores stock the hardcover edition.

And this book isn’t even literature in the strictest sense.

“Faithful,” the chatty, fan diary about the Red Sox 2004 season, has celebrated its seventh week among the best sellers on the New York Times’ list of hardcover nonfiction.

For writer Stewart O’Nan, the success of “Faithful” is part fate, built on the Sox’ unlikely championship, and partly due to his writing partner, Stephen King.

“That’s what happens when you’re writing with the King,” O’Nan said.

The duo, friends for the past few years, teamed up last winter. Together, they attended the Red Sox spring training camp in Florida. Then, they followed the team all season, from the first games till the last.

Their key was writing about each game when it was over, what O’Nan describes as a “memory journal.”

At its essence, the book replicates the emotional ride of baseball’s long, long season: 162 games from April to October.

Of course, the Red Sox played more, adding 14 post-season games and a world championship.

In a phone interview from his home in Avon, Conn., O’Nan still sounds jubilant about the Sox win. And a little grateful.

The season didn’t always look like a winner. As the Red Sox hit the doldrums of June and July, they lost as often as they won.

“The people who published the book were nervous, really nervous,” he said. They wondered what kind of book they’d get if the Sox didn’t at least reach the playoffs.

Meanwhile, the manuscript was growing.

Their process was to individually write something after each game. O’Nan’s job was to compile the copy, sometimes editing out long-winded bits and sending them off to Scribner’s editors in New York.

There, it was cut again.

“If they published everything we wrote, the book would have been 1,400 pages long,” O’Nan said. Each author wrote his own passages, everything from a description of a particular play on the field to a critique of the TV announcers. Every day, O’Nan would type their words into manuscript form, sometimes marveling at King’s musings.

“I’d think to myself, I am in the mind of Stephen King,'” he said.

Besides the essay-style writing, the book also features e-mail conversations between O’Nan, 43, and King, 57. Only a few of those are in the book.

“We did that a lot, a lot, a lot,” O’Nan said. Only about one-twentieth of that material made it into the final book, which has 414 pages.

Much of the season’s details survived, though.

All of the expected moments – such as Red Sox catcher Jason Varitek’s tussle with New York Yankees third baseman Alex Rodriguez – remain. But lots of whimsical moments are there, too. Many are based on O’Nan’s attendance at games.

O’Nan became particularly visible as the “net guy” after he brought a fishing net to Fenway Park and began scooping up balls during batting practice. He discovered that he could capture eight or nine per game by plucking them from his seat beside left field.

Sox utility man Gabe Kapler even borrowed the net once during practice, using it to shag balls. After six or seven games, the Sox finally banned it, though.

In all, O’Nan figures he attended 50 to 60 games. King attended fewer. They paid for every ticket themselves, adamant to be independent of the team or the press.

Of course, King’s seats were better than O’Nan’s. The latter writer sat wherever he could, sometimes using King’s seats when he wasn’t in town. The famous author had season tickets that O’Nan described as “dream seats,” two rows back from the field and behind home plate.

“You felt like you were part of the team,” he said of the view.

He still does.

“I think the experience of working on the book has made me even more of a fan,” said O’Nan, who plans to return to spring training in February. Yet, unlike many fans who bought championship DVDs, pennants and hats, he didn’t.

His favorite item is a 2004 Spring Training ball cap with an embroidered palm tree.

“It tells people I was there from the beginning,” he said.